Hoods and suction blowers are used to collect vapors, greases and odors from ranges and cooking units and discharge these materials to the outside atmosphere. The hoods are located above the range and ovens and have fans which draw the cooking pollutants through the hoods of the outside of the structure. In commercial installations large amounts of smoky and grease-laden air is discharged to the atmosphere. This practice results in large heat losses and air pollution. Ventilating systems which utilize filters and blowers to recirculate the air through the cooking unit have been developed. Examples of these systems are shown by Jenson in U.S. Pat. No. 3,260,189 and Gaylord in U.S. Pat. No. 3,785,124.